This invention relates to a inductive-discharge ignition device for an internal combustion engine.
More particularly, the invention relates to a device of the type comprising, for each spark plug, a transformer with a secondary winding to be connected to a respective spark plug and a primary winding connected to the output of a power stage able to generate an ON-OFF signal for controlling the transformer.
The power stage is generally provided at its output with a Darlington-connected transistor the collector of which is directly connected to a terminal of said transformer primary winding, the other primary winding terminal being connected to a direct current power source such as an accumulator battery.
The operation of known devices of the aforesaid type is essentially to provide the transformer primary winding with an alternating signal by putting said power transistor alternately into a conducting and inhibited state. By this means, a high voltage is induced (especially following the inhibition of the transistor) in the transformer secondary winding of such an extent as to trigger an arc in the respective spark plug.
It has also been noted that particularly during the OFF-ON switching stage, a damped oscillating signal is superposed on the voltage induced in the secondary winding, this signal being of high frequency (depending not only on the transformer parameters but also on the switching speed) and having the amplitude of its first half-wave directly proportional to the transformation ratio (turn ratio) of the transformer and to the direct current voltage with which said power transistor and primary winding are powered. Under certain engine operating conditions said oscillating signal can trigger the arc in the spark plug, resulting in considerably misplaced spark advance.
To obviate the said drawback it is known to provide the secondary winding with a series-connected diode (with the cathode facing the winding), the purpose of which is essentially to cut the unrequired high voltage.
The aforesaid method is however particularly expensive because of the very high cost of the diode used, which has to be chosen from those able to withstand a very high inverse voltage.